Apparatus for producing parts by selective sintering

ABSTRACT

An apparatus for selectively sintering a layer of powder to produce a part made from a plurality of sintered layers. The apparatus includes a computer controlling a laser to direct the laser energy onto the powder to produce a sintered mass. The computer either determines or is programmed with the boundaries of the desired cross-sectional regions of the part. For each cross-section, the aim of the laser beam is scanned over a layer of powder and the beam is switched on to sinter only the powder within the boundaries of the cross-section. Powder is applied and successive layers sintered until a completed part is formed. Preferably, the powder dispensing mechanism includes a drum which is moved horizontally across the target area and counter-rotated to smooth and distribute the powder in an even layer across the target area. A downdraft system provides controlled temperature air flow through the target area to moderate powder temperature during sintering.

The present application is a continuation of application Ser. No.07/911,879, filed Jul. 10, 1992 now U.S. Pat. No. 5,376,580, which is adivisional of application Ser. No. 541,788, filed Jun. 21, 1990 now U.S.Pat. No. 5,132,143, which is a divisional of application Ser. No.105,316, filed Oct. 5, 1987 now abandoned, which is continuation-in-partof Ser. No. 06/920,580, filed Oct. 17, 1986, now U.S. Pat. No.4,863,538.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method and apparatus which uses a directedenergy beam to selectively sinter a powder to produce a part. Inparticular, this invention relates to a computer aided laser apparatuswhich sequentially sinters a plurality of powder layers to build thedesired part in a layer-by-layer fashion. The present application isparticularly directed towards a mechanism for dispensing a layer ofpowder and a mechanism for directing air flow to the target area tomoderate powder temperature.

2. Description of the Relevant Art

The economies associated with conventional part production methods aregenerally related directly to the quantity of parts to be produced andthe desired material characteristics of the finished parts. For example,large scale manufacture casting and extrusion techniques are often costeffective, but these production methods are generally unacceptable forsmall quantities--i.e. replacement parts or prototype production. Manysuch conventional part production methods require expensive partspecific tooling. Even powder metallurgy requires a die for shaping thepowder, making powder metallurgy unattractive as a method for producinga small number of parts.

Where only a small number of parts are desired, conventional productionmethods involving a subtractive machining method are usually used toproduce the desired part. In such subtractive methods, material is cutaway from a starting block of material to produce a more complex shape.Examples of subtractive machine tool methods include: milling, drilling,grinding, lathe cutting, flame cutting, electric discharge machine, etc.While such conventional machine tool subtractive methods are usuallyeffective in producing the desired part, they are deficient in manyrespects.

First, such conventional machine tool subtractive methods produce alarge amount of waste material for disposal. Further, such machine toolmethods usually involve a large initial expense for setting up theproper machining protocol and tools. As such, the set-up time is notonly expensive, but relies a great deal on human judgment and expertise.These problems are, of course, exacerbated when only a small number ofparts are to be produced.

Another difficulty associated with such conventional machiningtechniques involves tool wear--which not only involves the cost ofreplacement, but also reduces machining accuracy as the tool wears.Another limit on the accuracy and tolerance of any part produced byconventional machining techniques is the tolerance limits inherent inthe particular machine tool. For example, in a conventional millingmachine or lathe, the lead screws and ways are manufactured to a certaintolerance, which limits the tolerances obtainable in manufacturing apart on the machine tool. Of course, the tolerances attainable arereduced with age of the machine tool.

The final difficulty associated with such conventional machine toolsubtractive processes is the difficulty or impossibility of making manypart configurations. That is, conventional machining methods are usuallybest suited for producing symmetrical parts and parts where only theexterior part is machined. However, where a desired part is unusual inshape or has internal features, the machining becomes more difficult andquite often, the part must be divided into segments for production. Inmany cases, a particular part configuration is not possible because ofthe limitations imposed upon the tool placement on the part. Thus, thesize and configuration of the cutting tool do not permit access of thetool to produce the desired configuration.

There are other machining processes which are additive, for example,plating, cladding, and some welding processes are additive in thatmaterial is added to a starting substrate. In recent years, otheradditive-type machining methods have been developed which use a laserbeam to coat or deposit material on a starting article. Examples includeU.S. Pat. Nos. 4,117,302; 4,474,861; 4,300,474; and 4,323,756. Theserecent uses of lasers have been primarily limited to adding a coating toa previously machined article. Often such laser coating methods havebeen employed to achieve certain metallurgic properties obtainable onlyby such coating methods. Typically, in such laser coating methods thestarting article is rotated and the laser directed at a fixed locationwith the coating material sprayed onto the article so that the laserwill melt the coating onto the article.

SUMMARY OF THE INVENTION

The problems outlined above are in large measure solved by the methodand apparatus of the present invention. The present invention includes adirected energy beam--such as a laser--and is adaptable to producealmost any three dimensional part. The method of the present inventionis an additive process, with the powder being dispensed into a targetarea where the laser selectively sinters the powder to produce asintered layer. The invention is a layer-wise process in which thelayers are joined together until the completed part is formed. Themethod of the present invention is not limited to a particular type ofpowder, but rather is adaptable to plastic, metal, polymer, ceramicpowders, or composite materials.

Broadly speaking, the apparatus includes a laser or other directedenergy source which is selectable for emitting a beam in a target areawhere the part is produced. A powder dispenser system deposits powderinto the target area. A laser control mechanism operates to move the aimof the laser beam and modulates the laser to selectively sinter a layerof powder dispensed into the target area. The control mechanism operatesto selectively sinter only the powder disposed within defined boundariesto produce the desired layer of the part. The control mechanism operatesthe laser to selectively sinter sequential layers of powder, producing acompleted part comprising a plurality of layers sintered together. Thedefined boundaries of each layer correspond to respectivecross-sectional regions of the part. Preferably, the control mechanismincludes a computer--e.g. a CAD/CAM system--to determine the definedboundaries for each layer. That is, given the overall dimensions andconfiguration of the part, the computer determines the definedboundaries for each layer and operates the laser control mechanism inaccordance with the defined boundaries. Alternatively, the computer canbe initially programmed with the defined boundaries of each layer.

In a preferred form, the laser control mechanism includes a mechanismfor directing the laser beam in the target area and a mechanism formodulating the laser beam on and off to selectively sinter the powder inthe target area. In one embodiment, the directing mechanism operates tomove the aim of the laser beam in a continuous raster scan of targetarea. The modulating mechanism turns the laser beam on and off so thatthe powder is sintered only when the aim of the laser beam is within thedefined boundaries for the particular layer. Alternatively, thedirecting mechanism aims the laser beam only within the definedboundaries for the particular layer so that the laser beam can be lefton continuously to sinter the powder within the defined boundaries forthe particular layer.

In a preferred embodiment, the directing mechanism moves the laser beamin a repetitive raster scan of the target area using a pair of mirrorsdriven by galvanometers. The first mirror reflects the laser beam to thesecond mirror which reflects the beam into the target area. Shiftingmovement of the first mirror by its galvanometer shifts the laser beamgenerally in one direction in the target area. Similarly, shiftingmovement of the second mirror by its galvanometer shifts the laser beamin the target area in a second direction. Preferably, the mirrors areoriented relative to each other so that the first and second directionsare generally perpendicular to each other. Such an arrangement allowsfor many different types of scanning patterns of the laser beam in thetarget area, including the raster scan pattern of the preferredembodiment of the present invention.

The method of part production of the present invention includes thesteps of depositing a first portion of powder onto a target surface,scanning the aim of a directed energy beam (preferably a laser) over thetarget surface, and sintering a first layer of the first powder portionon the target surface. The first layer corresponds to a firstcross-sectional region of the part. The powder is sintered by operatingthe directed energy source when the aim of the beam is within theboundaries defining the first layers. A second portion of powder isdeposited onto the first sintered layer and the aim of the laser beamscanned over the first sintered layer. A second layer of the secondpowdered portion is sintered by operating the directed energy sourcewhen the aim of the beam is within the boundaries defining the secondlayer. Sintering of the second layer also joins the first and secondlayers into a cohesive mass. Successive portions of powder are depositedonto the previously sintered layers, each layer being sintered in turn.In one embodiment, the powder is deposited continuously into the target.

In a preferred embodiment, the laser beam is modulated on and off duringthe raster scan so that the powder is sintered when the aim of the beamis directed within the boundaries of the particular layer. Preferably,the laser is controlled by a computer; the computer may include aCAD/CAM system, where the computer is given the overall dimensions andconfiguration of the part to be made and the computer determines theboundaries of each cross-sectional region of the part. Using thedetermined boundaries, the computer controls the sintering of each layercorresponding to the cross-sectional regions of the part. In analternative embodiment, the computer is simply programmed with theboundaries of each cross-sectional region of the part.

Additionally, another embodiment of the present invention includes adevice for distributing the powder as a layer over the target area orregion. Preferably, the distributing device includes a drum, a mechanismfor moving the drum across the region, and a mechanism forcounter-rotating the drum as it is moved across the region. The drummoving mechanism preferably keeps the drum a desired spacing above theregion to yield a layer of powder of a desired thickness. The drum isoperable when counter-rotated and moved across the region to projectpowder forward in the direction of movement, leaving behind a layer ofpowder having the desired thickness.

In still another embodiment, a downdraft mechanism for controllingtemperature of the powder is provided which includes a support definingthe target area, a mechanism for directing air to the target area, and amechanism for controlling the temperature of the air prior to reachingthe target area. The support preferably includes porous medium on whichthe powder is deposited and a plenum adjacent the porous medium. Thus,the controlled temperature air is directed to the powder in the targetarea and helps control the temperature of the sintered and unsinteredpowder in the target area.

As can be appreciated from the above general description, the method andapparatus of the present invention solves many of the problemsassociated with known part production methods. First, the presentinvention is well suited for prototype part production or replacementpart production of limited quantities. Further, the method and apparatushereof are capable of making parts of complex configurationsunobtainable by conventional production methods. Further, the presentinvention eliminates tool wear and machine design as limiting factors onthe tolerances obtainable in producing the part. Finally, with theapparatus of the present invention incorporated into a CAD/CAMenvironment, a large number of replacement parts can be programmed intothe computer and can be easily produced with little set-up or humanintervention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the apparatus of the presentinvention;

FIG. 2 is a schematic showing a portion of the layered build up of apart produced in accordance with the present invention, and illustratingthe raster scan pattern of the laser beam in the target area;

FIG. 3 is a block diagram depicting the interface hardware between thecomputer, laser and galvanometers of the present invention;

FIG. 4 is a perspective view of an example part produced in accordancewith the present invention;

FIG. 5 is a sectional view with parts broken away and in phantom, of thepart illustrated in FIG. 4;

FIG. 6 is a flow chart of the data metering program in accordance withthe present invention;

FIG. 7 is a sectional view taken along line 7--7 of FIG. 4;

FIG. 8 illustrates in diagram form the correlation between a singlesweep of the laser across the layer of FIG. 7 and the control signals ofthe present invention;

FIG. 9 is a schematic, vertical, sectional view of the powder dispensingdevice of the present invention distributing powder in a layer on thepart being produced;

FIG. 10 is a schematic, perspective view of the powder dispensing deviceof the present invention; and

FIG. 11 is an apparatus for moderating the temperature of the powder inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIG. 1 broadly illustrates the apparatus 10in accordance with the present invention. Broadly speaking, theapparatus 10 includes a laser 12, powder dispenser 14, and laser controlmeans 16. In more detail, the powder dispenser 14 includes a hopper 20for receiving the powder 22 and having an outlet 24. The outlet 24 isoriented for dispensing the powder to a target area 26, which in FIG. 1is generally defined by the confinement structure 28. Of course, manyalternatives exist for dispensing the powder 22.

The components of the laser 12 are shown somewhat schematically in FIG.1 and include a laser head 30, a safety shutter 32, and a front mirrorassembly 34. The type of laser used is dependent upon many factors, andin particular upon the type of powder 22 that is to be sintered. In theembodiment of FIG. 1, a Nd:YAG laser (Lasermetrics 9500Q) was used whichcan operate in a continuous or pulsed mode with a hundred-watt maximumoutlet power in the continuous mode. The laser beam output of the laser12 has a wavelength of approximately 1060 nM, which is near infrared.The laser 12 illustrated in FIG. 1 includes an internal pulse rategenerator with a selectable range of about one kilohertz to fortykilohertz, and an approximately six nanosecond pulse duration. In eitherthe pulsed or continuous mode, the laser 12 can be modulated on or offto selectively produce a laser beam which travels generally along thepath shown by the arrows in FIG. 1.

To focus the laser beam, a diverging lens 36 and converging lens 38 aredisposed along the path of travel of the laser beam as shown in FIG. 1.Using just the converging lens 38, the location of the true focal pointis not easily controlled by varying the distance between the converginglens 38 and the laser 12. The diverging lens 36 placed between the laser12 and converging lens 38 creates a virtual focal point between thediverging lens 36 and the laser 12. Varying the distance between theconverging lens 38 and-the virtual focal point, allows control of thetrue focal point along the laser beam path of travel on the side of theconverging lens 38 remote from the laser 12. In recent years there havebeen many advances in the field of optics, and it is recognized thatmany alternatives are available to efficiently focus the laser beam at aknown location.

In more detail, the laser control means 16 includes computer 40 andscanning system 42. In a preferred embodiment, the computer 40 includesa microprocessor for controlling the laser 12 and a CAD/CAM system forgenerating the data. In the embodiment illustrated in FIG. 1, a personalcomputer is used (Commodore 64) whose primary attributes include anaccessible interface port and a flag line which generates a nonmaskableinterrupt.

As shown in FIG. 1, the scanning system 42 includes a prism 44 forredirecting the path of travel of the laser beam. Of course, physicallayout of the apparatus 10 is the primary consideration in determiningwhether a prism 44, or a plurality of prisms 44, are needed tomanipulate the path of travel of the laser beam. The scanning system 42also includes a pair of mirrors 46, 47 driven by respectivegalvonometers 48, 49. The galvanometers 48, 49 coupled to theirrespective mirrors 46, 47 to selectively orientate the mirrors 46, 47.The galvanometers 46, 47 are mounted perpendicular to each other suchthat the mirrors 46, 47 are mounted nominally at a right angle to eachother. A function generator driver 50 controls the movement of thegalvanometers 48 (galvanometer 49 is slaved to the movement ofgalvanometer 48) so that the aim of the laser beam (represented by thearrows in FIG. 1) can be controlled in the target area 26. The driver 50is operatively coupled to the computer 40 as shown in FIG. 1. It will beappreciated that alternative scanning methods are available for use asthe scanning system 42, including acusto-optic scanners, rotatingpolygon mirrors, and resonant mirror scanners.

Turning to FIG. 2 of the drawing, a portion of a part 52 isschematically illustrated and shows four layers 54-57. The aim of thelaser beam, labeled 64 in FIG. 2, is directed in a raster scan patternas at 66. As used herein, "aim" is used as a neutral term indicatingdirection, but does not imply the modulation state of the laser 12. Forconvenience, the axis 68 is considered the fast scan axis, while theaxis 70 is referred to as the slow scan axis. Axis 72 is the directionof part build-up.

Turning to FIGS. 9 and 10, an alternative form of powder dispenser 20 isillustrated. Broadly speaking, a support defines a target area 102 wherethe aim of the beam 64 is directed (see FIG. 1). A hopper 104 dispensesthe powder 106 through opening 108 into the target area 102. A meteringroller (not shown) is disposed in the opening 108, such that whenrotated the metering roller deposits a metered mound of powder in a lineat end 110 of the target area 102.

A leveling mechanism 114 spreads the mound of powder 106 from end 110 tothe other end 112 of the target area. The leveling mechanism 114includes a cylindrical drum 116 having an outer knurled surface. A motor118 mounted on bar 120 is coupled to the drum 116 via pulley 122 andbelt 124 to rotate the drum.

The leveling mechanism 114 also includes a mechanism 126 for moving thedrum 116 between end 110 and end 112 of the target area. The mechanism126 comprises an X/Y table for moving the bar 120 horizontally andvertically. That is, table 128 is fixed while plate 130 is selectivelymoveable relative thereto.

Still another embodiment is shown in FIG. 11 for controlling thetemperature of the article being produced. Undesirable shrinkage of thearticle being produced has been observed to occur due to differencesbetween the temperature of the particles not yet scanned by the directedenergy beam and the temperature of the previously scanned layer. It hasbeen found that a downward flow of controlled-temperature air throughthe target area is able to moderate such undesirable temperaturedifferences. The controlled-temperature air downdraft system 132 of FIG.11 reduces thermal shrinkage by providing heat transfer between thecontrolled-temperature air and the top layer of powder particles to besintered. This heat transfer moderates the temperature of a the toplayer of particles to be sintered, controls the mean temperature of thetop layer, and removes bulk heat from the article being produced,thereby reducing its bulk temperature and preventing the article fromgrowing into the unsintered material. The temperature of the incomingair is adjusted to be above the softening point of the powder, but belowthe temperature at which significant sintering will occur.

The downdraft system 132 broadly includes a support 134 defining targetarea 136, means for directing air to the target area, and a mechanismfor controlling the temperature of the incoming air, such as resistanceheater 142. The air directing means includes chamber 138 surroundingsupport 134, fan 140 and/or vacuum 141. A window 144 admits the aim ofthe beam 64 (FIG. 1) to the target area 136. A powder dispensingmechanism (not shown), such as illustrated in FIGS. 1 or 10 is disposedat least partially in the chamber 138 to dispense powder onto targetarea 136.

Support 134 preferably comprises a filter medium 146 (such as asmall-pore paper) on top of a honey-comb porous medium 148. A plenum 150is disposed for gathering air for passage to outlet 152. Of course, theoutlet 152 is connected to vacuum 141 or other air handling mechanism.

Operation

A fundamental concept of the present invention is the build up of a partin a layer-by-layer manner. That is, a part is considered a plurality ofdiscrete cross-sectional regions which cumulatively comprise thethree-dimensional configuration of the part. Each discretecross-sectional region has defined two-dimensional boundaries--ofcourse, each region may have unique boundaries. Preferably, thethickness (dimension in the axis 72 direction) of each layer isconstant.

In the method, a first portion of powder 22 is deposited in the targetarea 26 and selectively sintered by the laser beam 64 to produce a firstsintered layer 54 (FIG. 2). The first sintered layer 54 corresponds to afirst cross-sectional region of the desired part. The laser beamselectively sinters only the deposited powder 22 within the confines ofthe defined boundaries.

There are, of course, alternative methods of selectively sintering thepowder 22. One method is for the aim of the beam to be directed in a"vector" fashion--that is, the beam would actually trace the outline andinterior of each cross-sectional region of the desired part.Alternatively, the aim of the beam 64 is scanned in a repetitive patternand the laser 12 modulated. In FIG. 2, a raster scan pattern 66 is usedand is advantageous over the vector mode primarily in its simplicity ofimplementation. Another possibility is to combine the vector and rasterscan methods so that the desired boundaries of the layer are traced in avector mode and the interior irradiated in a raster scan mode. Thereare, of course, trade-offs associated with the method chosen. Forexample, the raster mode has a disadvantage when compared to the vectormode in that arcs and lines which are not parallel to the axes 68, 70 ofthe raster pattern 66 of the laser beam 64 are only approximated. Thus,in some cases resolution of the part can be degraded when produced inthe raster pattern mode. However, the raster mode is advantageous overthe vector mode in the simplicity of implementation.

Turning to FIG. 1, the aim of the laser beam 64 is scanned in the targetarea 26 in a continuous raster pattern. Broadly speaking, the driver 50controls galvanometers 48, 49 to made the raster pattern 66 (see FIG.2). Shifting movement of the mirror 46 controls movement of the aim ofthe laser beam 64 in the fast scan axis 68 (FIG. 2), while movement ofthe mirror 47 controls movement of the aim of the laser beam 64 in theslow scan access 70.

The present position of the aim of the beam 64 is fed back through thedriver 50 to the computer 40 (see FIG. 3). As described below, in moredetail, the computer 40 possesses information relating to the desiredcross-sectional region of the part then being produced. That is, aportion of loose powder 22 is dispensed into the target area 26 and theaim of the laser beam 64 moved in its continuous raster pattern. Thecomputer 40 modulates the laser 12 to selectively produce a laser beamat desired intervals in the raster pattern 66. In this fashion, thedirected beam of the laser 12 selectively sinters the powder 22 in thetarget area 26 to produce the desired sintered layer with the definedboundaries of the desired cross-sectional region. This process isrepeated layer-by-layer with the individual layers sintered together toproduce a cohesive part--e.g. part 52 of FIG. 2.

Because of the relatively low output power of the laser head 30illustrated in FIG. 1, the powder 22 consisted of a plastic material(e.g. ABS), based on the lower heat of fusion of most plastics, which iscompatible with the lower power laser. Several post formation treatmentsare contemplated for the parts produced by the apparatus 10 of thepresent invention. For example, if such a produced part is to be usedonly as a prototype model or as a die for sandcast or lost wax casting,then post-formation treatment may not be necessary. In some situations,certain surfaces of the parts produced may be designed for closetolerances, in which case some post-fabrication machining would beaccomplished. Alternatively, some types of parts may require certainmaterial properties which can be achieved by heat-treating and/orchemically treating the part. For example, the granule size of thepowder 22 could be such to produce a part having an open porosity andepoxy or similar substance injected into the part could achieve thedesired material properties--e.g. compression strength, abrasionresistance, homogeneity, etc.

Several characteristics of powder 22 have been identified which improveperformance. First, energy absorption by the powder can be controlled bythe addition of a dye such as carbon black. Adjusting the concentrationand composition of the additive controls the absorbtivity constant K ofthe powder. Generally, energy absorptivity is governed by theexponential decay relation:

    I(z)=I.sub.o exp (K Z)

where I(z) is the optical intensity (powder per unit area) in the powderat a distance z normal to the surface, I_(o) is the surface value of I(intensity at the surface), and K is the absorptivity constant.Adjustment of the absorptivity constant K and adjustment of the layerthickness in which a given fraction of beam energy is absorbed givesoverall control of the energy absorbed in the process.

Another important characteristics of the powder is the aspect ratio ofthe particles (i.e. ratio of the particle's maximum dimension to itsminimum dimension). That is, particles with certain aspect ratios tendto warp during shrinkage of the part. With particles having low aspectratios, i.e. nearly spherical, part shrinkage is more three dimensional,resulting in greater warp. When particles with high aspect ratios areused (e.g. flakes or rods) shrinkage primarily is in a verticaldirection reducing or eliminating warping of the part. It is believedthat high aspect ratio particles have greater freedom to accommodatebonding and interparticle contact is preferentially oriented inhorizontal planes causing shrinkage to occur primarily in a verticaldirection.

Turning now to FIGS. 9 and 10, the dispensing mechanism 114 has beenfound to provide a controlled level layer of powder in the target area102 without disturbing the part being produced. A metered amount ofpowder 106 is deposited at end 110 of the target area 102. The drum 116is spaced away from end 110 when the powder 106 is dispensed. In thesystem illustrated in FIG. 10, the plate 130 and bar 120 (and attachedmechanisms) are raised vertically after the powder is dispensed in themound. Travel of the plate 130 towards the hopper 104 brings the drum116 into position adjacent the mound of powder lined up along end 110.The drum 116 is lowered to contact the mound of powder and broughthorizontally across the target area 102 to spread the mound of powder ina smooth even layer. Of course, the precise location of plate 130relative to table 128 can be controlled, so that the spacing betweendrum 116 and target area 102 can be precisely controlled to yield thedesired thickness to the layer of powder. Preferably, the spacingbetween the drum 116 and target area 102 is constant to give a parallelmotion, but other spacing options are available.

As the drum 116 is moved horizontally across the target area 102 fromend 110 to end 112, motor 118 is activated to counter-rotate the drum116. As shown in FIG. 9, "counter-rotation" means the drum 116 isrotated in the direction R counter to the direction of movement M of thedrum 116 horizontally across the target area 102.

In more detail (FIG. 9), the drum 116 is counter-rotated at high speedto contact the mound of powder 106 along the trailing edge 160. Themechanical action of the drum on the powder ejects the powder to thedirection of movement M so that the ejected particles fall in the regionof the leading edge of the powder 162. As illustrated in FIG. 9, asmooth, level layer of powder is left behind the drum 116 (between drum116 and end 110) as depicted at 164.

FIG. 9 also illustrates schematically that the powder 106 can bedistributed over the target area without disturbing previously sinteredpowder 166 or unsintered powder 168. That is, the drum 116 is movedacross the target area 102 without transmitting shear stress to thepreviously built up layers and without disturbing the article as it isbeing produced. The absence of such sheer stress permits a smooth layerof powder 106 to be distributed on the fragile substrate in the targetarea, which includes both the sintered particles 166 and the unsinteredparticles 168.

Interface and Software

The interface hardware operatively interconnects the computer 40 withthe laser 12 and galvanometers 47, 48. The output port of the computer40 (see FIGS. 1 and 3) is directly connected to the laser 12 toselectively modulate the laser 12. When operated in the pulsed mode, thelaser 12 is easily controlled by digital inputs to the pulsed gate inputof the laser. Galvanometer 48 is driven by the function generator driver50 to drive the beam in the fast scan axis 68 independent of any controlsignals from the computer 40. However, a position feedback signal fromthe galvanometer 48 is fed to a voltage comparator 74 as shown in FIG.3. The other input to the comparator is connected to thedigital-to-analog convertor 76 which is indicative of the leastsignificant six bits (bits 0-5) of the user port of the computer 40. Asshown in FIG. 3, the output of the voltage comparator 74 is connected tothe flag line on the user port of the computer 40. When the voltagecomparator determines that the feedback signal from the galvanometer 48crosses the signal from the digital-to-analog convertor 76, the flagline goes low causing a nonmaskable interrupt. As discussed below, thenonmaskable interrupt causes the next byte of data to put out on theuser port of a computer 40.

Finally, as shown in FIG. 3, the galvanometer 49 driving the aim of thelaser beam 64 in the slow scan axis 70, is controlled by a seconddigital to analog convertor 78. The digital-to-analog convertor 78 isdriven by a counter 79 which increments with each sweep of the aim ofthe beam 64 in the fast scan axis 68. The eight byte counter is designedto overflow after 256 scans in the fast scan axis 68 to start a newcycle or raster scan pattern 66.

Preferably, the control information (i.e. defined boundaries of thecross-sectional regions) data for each raster pattern 66 would bedetermined by a CAD system given the overall dimensions andconfiguration of the part to be produced. Whether programmed or derived,the control information data for each raster pattern 66 is stored in thecomputer memory as a series of eight bit words. The data formatrepresents a pattern of "on" and "off" regions of the laser 12, versusdistance along the raster pattern 66 traveled by the aim of the beam 64.The data is stored in a "toggle-point" format where the data representsthe distance along each raster scan pattern 66 where the laser ismodulated (i.e. turned from on to off or from off to on). Although a"bit map" format might be used, the toggle point format has been foundmore efficient for the production of high resolution parts.

For each eight bit word, the least significant six bits (bits 0-5)represent the location of the next toggle point--i.e. the next locationfor modulation of the laser 12. The next bit (bit 6) represents whetherthe laser is on or off immediately before the toggle point identified inthe least significant six bits. The most significant bit (MSB or bit 7)is used for looping and for controlling the slow scan axis 70 of the aimof the beam 64. Because the Commodore 64 had limited memory, looping wasrequired--it being understood that a computer 40 with more memory wouldnot require looping.

FIG. 6 represents the flow chart for the data metering program. The datametering program is run whenever the flagline goes low causing anon-maskable interrupt (see FIG. 3). The interrupt causes themicroprocessor of the computer 40 to retrieve a two byte interruptvector which points to the location in memory where program control istransferred at interrupt. As shown in FIG. 6, the data metering programfirst pushes the registers onto the stack and then loads the next byteof data into the accumulator. The data word is also output to the userport with the sixth bit used to modulate the laser 12 (FIG. 3).

As shown in FIG. 6, the most significant bit (MSB or bit 7) of the dataword in the accumulator is examined. If the value of the mostsignificant bit is one, that means the end of the loop has not beenreached; therefore the data pointer is incremented, registers arerestored from the stack, and the data metering program is exited,returning control to the microprocessor at the location of interrupt. Ifthe most significant bit in the accumulator is zero, the data word isthe last word in the loop. If the data word is the last word in theloop, the next bit in memory is a loop counter and the following twobytes are a vector pointing to the top of the loop. As can be seen fromFIG. 6, if the most significant bit equals zero (end of the loop) theloop counter (next bit) is decremented and analyzed. If the loop counteris still greater than zero, the data pointer assumes the value from thenext two memory bytes after the loop counter, registers are pulled fromthe stack and program control returns to the location of interrupt. Onthe other hand, if loop counter is zero, the data pointer is incrementedby three and the loop counter is reset to ten before exiting theprogram. It can be appreciated that the need for such looping isabsolved if the memory size of the computer 40 is adequate.

EXAMPLE

In FIGS. 4 and 5, an example part 52 is illustrated. As can be seen fromthe drawing, the example part 52 assumes an unusual shape in that it isnot symmetrical and would be difficult to fabricate using conventionalmachining methods. For reference purposes, the part 52 includes an outerbase structure 80 having an interior cavity 82 and a pillar 84 disposedwithin the cavity 82 (see FIG. 4). FIG. 5 shows the part 52 within theconfinement structure 28 defining the target area 26 illustrated inFIG. 1. As shown in FIG. 5, some of the powder 22 is loose, while theremainder of the powder is selectively sintered to comprise thestructure of the part 52. FIG. 5 is shown in vertical section with partsbroken away and outlined in phantom to show the sintered cohesiveportions of the part 52.

FIG. 7 shows a horizontal cross-sectional region, taken along line 7--7of FIG. 4. FIG. 7 represents a discrete layer 86 associated with thecross-sectional region of the part being produced. As such, the sinteredlayer 86 of FIG. 7 is a product of a single raster pattern 66 asillustrated in FIG. 2.

For reference purposes, a sweep line through the sintered layer 86 hasbeen labeled "L." FIG. 8 illustrates the software and hardware interfaceoperation during the sweep L. The top graph shows the position offeedback signal from the fast axis galvo 48 and the output signal of thefirst digital to analog convertor 76 (compare FIG. 3). The voltagecomparator 74 generates an output signal to the flag line of thecomputer 40 every time the feedback signal and first D/A output signalcross.

In the top graph of FIG. 8, these points are labeled T to representtoggle points. As can be seen from the bottom graph of FIG. 8, the flagline generates a nonmaskable interrupt corresponding to each togglepoint T. The sixth bit of each data word is analyzed and the currentstate of the laser 12 will reflect the value. The penultimate graph ofFIG. 8 shows the laser modulation signal for the sweep line L of FIG. 7.The second graph of FIG. 8 shows that a high-going edge in the mostsignificant bit will be encountered at the end of each sweep of the aimof the laser beam 64 in the fast scan axis 68. As shown in FIGS. 3 and6, the counter 79 increments on a high going edge, and outputs a signalto the second digital-analog convertor 78 to drive the slow axisgalvanometer 49.

As can be seen from the example illustrated in the drawing, parts ofcomplex shape can be produced with relative ease. Those skilled in theart will appreciate that the part 52 illustrated in FIG. 4 would bedifficult to produce using conventional machining methods. Inparticular, machine tool access would make the fabrication of cavity 82and pillar 84 difficult, if not impossible, to produce if the part 52were of a relatively small size.

In addition to avoiding the access problem, it will be appreciated thatthe production accuracy is not dependent upon machine tool wear and theaccuracy of mechanical components found in conventional machine tools.That is, the accuracy and tolerances of the parts produced by the methodand apparatus of the present invention are primarily a function of thequality of the electronics, the optics, and the implementing software.Of course, heat transfer and material considerations do affect thetolerances obtainable.

Those skilled in the art will appreciate that conventional machiningtechniques require considerable human intervention and judgment. Forexample, a conventional machining process, such as milling, wouldrequire creativity to make such decisions as tool selection, partsegmenting, sequence of cuts, etc. Such decisions would even be moreimportant when producing a control tape for a tape control millingmachine. On the other hand, the apparatus of the present invention onlyrequires the data relating to each cross-Sectional region of the partbeing produced. While such data can be simply programmed into thecomputer 40, preferably, the computer 40 includes a CAD/CAM system. Thatis, the CAD/CAM portion of the computer 40 is given the overalldimensions and configurations of the desired part to be produced and thecomputer 40 determines the boundaries for each discrete cross-sectionalregion of the part. Thus, a vast inventory of part information can bestored and fed to the computer 40 on a selectable basis. The apparatus10 produces a selected part without set-up time, part specific tooling,or human intervention. Even the complex and expensive dies associatedwith powder metallargy and conventional casting techniques are avoided.

While large quantity production runs and certain part materialcharacteristics might be most advantageously made using conventionalfabrication techniques, the method and apparatus 10 of the presentinvention is useful in many contexts. In particular, prototype modelsand casting patterns are easily and inexpensively produced. For example,casting patterns are easily made for use in sand casting, lost waxcasting, or other forming techniques. Further, where desired quantitiesare very small, such as with obsolete replacement parts, production ofsuch replacement parts using the apparatus 10 of the present inventionhas many advantages. Finally, the use of the apparatus 10 may be usefulwhere size of production facilities is a major constraint, such ason-ship or in outerspace.

I claim:
 1. An apparatus for producing a part from a powder,comprising:means for successively dispensing a plurality of layers ofpowder at a target surface; an energy source; a controller for directingthe energy source at locations of each dispensed layer of powder at thetarget surface corresponding to cross-sections of the part to beproduced therein and fusing the powder thereat; and temperature controlmeans for moderating the temperature difference between unfused powderin a topmost layer of powder at the target surface and fused powder inthe one of the plurality of layers of powder immediately beneath thetopmost layer.
 2. The apparatus of claim 1, wherein said temperaturecontrol means comprises:a heater for heating a gas; and means fordirecting the heated gas at the target surface.
 3. The apparatus ofclaim 2, wherein said temperature control means furthercomprises:exhaust means for exhausting directed heated gas from thevicinity of the target surface.
 4. The apparatus of claim 1, whereinsaid energy source comprises a laser;and wherein said controllercomprises:a computer; and mirrors controlled by said computer to directthe aim of the beam from the laser.
 5. The apparatus of claim 4, whereinsaid controller further comprises:interface hardware, coupled to saidcomputer, to turn on and off the laser as its aim is moved across thetarget surface.
 6. The apparatus of claim 5, wherein the computer isprogrammed with the defined boundaries of each cross-section of thepart.
 7. The apparatus of claim 5, wherein the computer comprises meansfor determining the defined boundaries of each layer of the part fromthe overall dimensions of the part.
 8. The apparatus according to claim1, wherein the dispensing means comprises:means for dispensing powdernear said target surface; a drum; means for moving said drum across saidtarget surface in contact with said powder; and means for rotating saiddrum counter to a direction of said movement of said drum across saidtarget surface; wherein said movement and said counter-rotation of saiddrum distribute a layer of powder over said target surface.
 9. Anapparatus for producing a part from a powder, comprising:means fordispensing powder at a target surface; an energy source; a controllerfor directing the energy source at locations of powder at the targetsurface corresponding to cross-sections of the part to be produced andfusing the powder thereat; a heater for heating a gas; means fordirecting the heated gas at the target surface; and exhaust meansdisposed below the target surface for flowing directed heated gasthrough the powder at the target surface and for exhausting directedheated gas from the vicinity of the target surface.
 10. An apparatus forproducing a part from a powder, comprising:means for dispensing powderat a target surface; an energy source; a controller for directing theenergy source at locations of powder at the target surface correspondingto cross-sections of the part to be produced and fusing the powderthereat; and temperature control means for moderating the temperaturedifference between unfused powder at the target surface and thecross-section of the part immediately therebeneath, comprising:a heaterfor heating a gas; means for directing the heated gas at the targetsurface; and exhaust means disposed below the target surface, forexhausting directed heated gas from the vicinity of the target surfaceso that heated gas flows through the powder at the target surface.